With ongoing improvements in the digitization of information, data embedding methods as represented by the electronic watermark method have come into common use, and have come to be applied to the analog image medium such as print-matters and reproductions in recent years.
The data embedding method for print-matters is represented by a technique, wherein, for example, information is divided into a plurality of blocks (hereinafter referred to as “information blocks”), and the information blocks are assigned with block numbers, whereby the information is embedded into the background of a document image and others at the time of printing (Patent Literature 1).
According to the method proposed in the Patent Literature 1, however, when the embedded information is read out, the data or a part thereof to be read our will be missing if the block number cannot be identified. Because of this problem, sufficient information detection performance cannot be achieved in an unreadable image wherein the background is read only in a limited area as exemplified by the document wherein letters are arranged over the entire surface.
A solution to this problem is proposed in a technique wherein the information block data wherein the block number cannot be identified is compared with other information block data wherein the block number has been identified, and the block is estimated as being the same information block as the one having a higher degree of agreement (Patent Literature 2).    Patent Literature 1: Japanese Unexamined Patent Application Publication No. 2004-112607    Patent Literature 2: Japanese Unexamined Patent Application Publication No. 2004-228739